Modification or treatment of a surface by applying glow plasma is a known technique in industries, such as photo film production industry, used in order to improve certain surface and material properties. For instance, in the production of photo film, a thermoplastic polymer film (polyethyleneterephthalate or polyethylene-naphthalate, or similar) is prepared using a glow plasma in order to improve adhesion properties of the surface.
Plasma is considered generally as a suitable solution for material processing, because it generates a large flux of reactive species (radicals, ions), which can be directed to the process zone and manipulated to the desired shape by using an appropriate electric field distribution. Plasma treatment would have considerable advantage if it could be generated at atmospheric pressure, and in the presence of air. Advantages of using atmospheric pressure are a larger density of reactive species than in the low pressure case, and the advantage of avoiding vacuum technology. The advantages of using air instead of other gasses is the fact that it is cheap and readily available.
Another desired feature of atmospheric pressure glow plasmas (APG) is the generation of these plasmas at low temperatures around 300-400 K. This will make the technology applicable to the treatment of thermoplastic polymer surfaces, as is common in photo film production methods.
Generating a plasma under the above circumstances is not a straight forward technique. At atmospheric pressure, the particle density is high and as a result the mean free path of reactive species is small. The processes of excitation and ionisation are restricted to a limited area, and the plasma is generated primary in a filamentary form.
Plasmas at atmospheric pressures are very unstable and will tend to go into a spark or an arc in short time after the breakdown. Any random local increase in a current density will tend to grow rather than to be damped and plasma will be constricted.
Transition to spark or arc can be prevented by limiting the current density and the plasma duration, by using high gas flows. The simplest solution to limit the current density and plasma duration is to cover the electrodes with a dielectric (dielectric barrier discharge configuration, DBD). As a result of this, the charges are accumulated on the surface of the dielectric, reducing the value of the voltage applied to the plasma. When the magnitude of the voltage applied to the plasma decreases below a critical level (the cut-off voltage), the plasma can not be sustained any longer. As a result, the duration of the plasma is limited.
A problem of dielectric barrier discharge is that by covering electrodes with a dielectric, high voltages for plasma ignition (2-10 kV) are required. The use of these high voltages is required to increase the ionisation degree, since a major source of electrons, the electron emission from the metal electrodes via ion bombardment or photo ionisation processes, is lost. As a result, most of the dielectric barrier plasmas are generated through a mechanism known as streamer formation. Streamers are short life, high electron density, filamentary plasmas. Due to non-uniformity of these plasmas and material damage as a result of the intense ion bombardment, application of this technique in material processing methods creates problems. Electronic means for suppressing streamer formation and generation of a uniform glow plasma are not available, and thus alternatives have been investigated.